In recent years, hydrofluorocarbons (HFCs) with zero ozone depletion potential and lower global warming potential (GWP) have been used as refrigerants for refrigerating machines. Among them, difluoromethane has a GWP as low as 675 and thus is a refrigerant preferred from the viewpoint of environmental problems. As refrigerating machine oils for the above refrigerants, International Publication Nos. WO2012/026303 and WO2013/062058 disclose ester-based refrigerating machine oils.
On the other hand, Japanese Patent Application Publication No. 2014-224271 teaches the following points of difluoromethane. Specifically, difluoromethane has poorer miscibility with refrigerating machine oils than other fluorine-based refrigerants, and accordingly tends to cause separation of the refrigerant from the oil. In such a case, sliding portions of a compressor constituting an air conditioner are supplied with only the refrigerant having a low lubricity, and thereby abnormal heat generation caused by the sliding portions of the compressor promotes acid generation due to the refrigerant degradation. In addition, since difluoromethane is less stable than the other fluorine-based refrigerants, it easily degrades when exposed to a high-temperature environment or mixed with air and water, and even worse generates a large amount of acids such as hydrofluoric acid when degraded. For these reasons, there are risks that the acids generated by the degradation of the refrigerant may deteriorate the refrigerating machine oil used in the refrigerating machine, and corrode parts such as an expansion valve. Still further, in the compressor of the refrigerating machine, difluoromethane is used under higher pressure than the other fluorine-based refrigerants. Accordingly, a large load tends to be applied to sliding portions of the compressor that compresses difluoromethane and the refrigerating machine oil forms a thin film between the surfaces of the sliding parts, with the result that wear and seizure easily occur. For the reasons mentioned above, the refrigerating machine oil used together with difluoromethane is required to have excellent miscibility with difluoromethane, excellent stability, and excellent lubricity.
Meanwhile, as a material for improving the lubricity, U.S. Pat. No. 5,017,300 discloses refrigerating machine oil compositions containing a polyacrylic ester for a HFC refrigerant selected from 1,1,1,2-tetrafluoroethane (R134a), 1,1,2,2-tetrafluoroethane (R134), and pentafluoroethane (R125). Japanese Patent Application Publication No. H04-275397 discloses refrigerating machine oil compositions containing a polyacrylic ester for an R134a refrigerant. However, the aforementioned literatures neither describe a composition containing a polyacrylic ester and an ester of a polyhydric alcohol with an aliphatic monocarboxylic acid, nor describe use of the aforementioned refrigerating machine oil compositions as those for a difluoromethane refrigerant. Japanese Patent Application Publication No. 2007-204568 discloses refrigerating machine oil compositions for a carbon dioxide refrigerant containing a base oil and an oil-soluble polymer. However, the foregoing literature does not describe use of the aforementioned refrigerating machine oil compositions as one for a difluoromethane refrigerant. In addition, Japanese Patent No. 3051673 discloses refrigerating machine oil compositions containing a Michael addition product of an acrylic ester and a malonic acid ester, and an ester oil.